Its not that Microsoft has nothing to gain from licensing its intellectual property. It has been reported in the past that companies that license IP from Microsoft pay it a few dollars on every device sold. Redmond may very well have a similar arrangement with the folks at Motorola Solutions.
Before Google bought and split it up, Motorola Mobility and Motorola Solutions used to be one company. Google bought the former for $12.5 billion back in January 2011, which primarily makes consumer focused smartphones and tablets. Motorola Solutions is more into enterprise tablets, wireless infrastructure equipment etc.
Interestingly Motorola Solutions recently sold a major chunk of its enterprise business to Zebra Technologies Corporations for $3.5 billion. It will retain its iDEN lineup though. On the other hand, even Google has sold Motorola Mobility for $3 billion to Lenovo.